An ultrasonic diagnostic apparatus is used to make a noninvasive checkup on a subject by irradiating him or her with an ultrasonic wave and analyzing the information contained in its echo signal. For example, a conventional ultrasonic diagnostic apparatus that has been used extensively converts the intensity of an echo signal into its associated pixel luminance, thereby presenting the subject's structure as a tomographic image. In this manner, the internal structure of the subject can be known. Meanwhile, an ultrasonic diagnostic apparatus for presenting subject's motion information such as blood flow information as an image by detecting Doppler shift in an echo signal has also been used.
Meanwhile, some people are attempting recently to track the motion of a subject's tissue more precisely and evaluate the strain and the modulus of elasticity, coefficient of viscosity or any other physical (attribute) property of the tissue mainly by analyzing the phase of the echo signal.
Patent Document No. 1 discloses a method for tracking a subject's tissue highly precisely and sensing very small vibrations of a cardiac tissue beating by determining the instantaneous location of the subject based on the amplitude and phase of the detected output signal of an echo signal. According to this method, a number of ultrasonic pulses are transmitted in the same direction toward a subject at regular intervals ΔT and the ultrasonic waves reflected from the subject are received. As shown in FIG. 32, the received echo signals are identified by y(t), y(t+ΔT) and y(t+2ΔT), respectively. Supposing the pulse transmission time t is 0, the receiving time t1 of an echo signal produced at a certain depth x1 is given by t1=x1/(C/2), where C is the sonic velocity. In this case, if the phase shift between y(t1) and y(t1+ΔT) is Δθ and the center frequency of the ultrasonic wave around t1 is f, the magnitude of displacement Δx of x1 during this period ΔT is calculated by the following Equation (1).Δx=−C·Δθ/4πf  (1)
The location x1′ of x1 in ΔT seconds can be figured out by adding the magnitude of displacement Δx to x1 as in the following Equation (2).x1′=x1+Δx  (2)By repeatedly performing this calculation, the same location x1 of the subject can be tracked. This method is called a “phased tracking method”.
Patent Document No. 2 further develops the method of Patent Document No. 1 into a method of calculating the modulus of elasticity of a subject's tissue (e.g., an arterial wall, in particular). According to this method, first, an ultrasonic wave is transmitted from a probe 101 toward a blood vessel wall 16 as shown in FIG. 33. And the echo signals, reflected from measuring points A and B on the blood vessel wall 16, are analyzed by the method of Patent Document No. 1, thereby tracking the motions of the measuring points A and B. FIG. 34 shows the tracking waveforms TA and TB of the measuring points A and B along with an electrocardiographic complex ECG.
As shown in FIG. 34, the tracking waveforms TA and TB have the same periodicity as the electrocardiographic complex ECG, which shows that the artery dilates and shrinks in sync with the cardiac cycle of the heart. More specifically, when the electrocardiographic complex ECG has outstanding peaks called “R waves”, the heart starts to shrink, thus pouring blood flow into the artery and raising the blood pressure. As a result, the blood vessel wall is dilated rapidly. That is why soon after the R wave has appeared on the electrocardiographic complex ECG, the artery dilates rapidly and the tracking waveforms TA and TB rise steeply, too. After that, however, as the heart dilates slowly, the artery shrinks gently and the tracking waveforms TA and TB gradually fall to their original levels. The artery repeats such a motion cyclically.
The difference between the tracking waveforms TA and TB is represented as a waveform W showing a variation in thickness between the measuring points A and B. The thickness change waveform W may also be regarded as a waveform representing strain between A and B. The greatest thickness change ΔW can be calculated as a difference between the maximum and minimum values Wmax and Wmin of the thickness change waveform W:ΔW=Wmax−Wmin  (3)
Supposing the reference thickness between the measuring points A and B during initialization is Ws, the magnitude of maximum strain ε between the measuring points A and B is calculated by the following Equation (4).ε=ΔW/Ws  (4)
Also, in this case, the highest and lowest blood pressures Pmax and Pmin of the subject are measured with a blood pressure manometer, for example. The blood pressure difference ΔP is given by the following Equation (5).ΔP=Pmax−Pmin  (5)
The magnitude of maximum strain ε should be caused by the blood pressure difference ΔP. As the modulus of elasticity Er is defined as a value obtained by dividing the stress by the strain, the modulus of elasticity Er between the measuring points A and B is given by the following Equation (6).
                                                        Er              =                            ⁢                                                Δ                  ⁢                                                                          ⁢                                      P                    /                    ɛ                                                  =                                  Δ                  ⁢                                                                          ⁢                                      P                    ·                                          Ws                      /                      Δ                                                        ⁢                                                                          ⁢                  W                                                                                                        =                            ⁢                              Δ                ⁢                                                                  ⁢                                  P                  ·                                      Ws                    /                                          (                                                                        W                          ⁢                                                                                                          ⁢                          max                                                -                                                  W                          ⁢                                                                                                          ⁢                          min                                                                    )                                                                                                                              (        6        )            
Non-Patent Document No. 1 discloses a method for calculating the modulus of elasticity of each portion based on the magnitude of maximum strain ε and the blood pressure difference ΔP in a situation where the blood vessel has non-uniform thicknesses.
Therefore, by making these calculations on multiple spots on a tomographic image, an image representing the distribution of elasticities Er can be obtained. If an atheroma 11 has been created in the blood vessel wall 16 as shown in FIG. 33, the atheroma and its surrounding blood vessel wall tissue have different elasticities. That is why if an image representing the distribution of elasticities is obtained, it can be determined how and where the atheroma has been produced.                Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 10-5226        Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 2000-229078        Non-Patent Document No. 1: Hasegawa et al., Evaluation of Regional Elastic Modulus of Cylindrical Shell with Non-Uniform Wall Thickness, J Med Ultrasonics, Vol. 28, No. 1 (2001)        